Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

Several Republicans repudiated Nugent’s slur. Sen. Ted Cruz, while stipulating that he wouldn’t say such a thing himself, declined to criticize Nugent and praised him as a compelling spokesman for gun rights.

There are lots of things we can learn from this episode. One is that Nugent is nuts, and it’s not a great idea to attach your party to him, as many Texas Republicans have done. Another is that Cruz is too cowardly to criticize anyone in his base.

The researchers found that when dogs were matched for age, sex, and weight, 13 of the 24 disorders showed “no significant difference in the mean proportion of purebred and mixed-breed dogs with the disorder.” Of the remaining 11 disorders, 10—including epilepsy, cataracts, hypothyroidism, and dilated cardiomyopathy—“were more prevalent in purebred dogs, compared with those found in mixed-breed dogs.” The only condition more common in mixed-breed dogs—by a margin of about 20 percent, much slimmer than the margins that went the other way—was “ruptured cranial cruciate ligament.”

Why are mutts healthier than purebreds? The prevailing theory is hybrid vigor. Many heritable disorders arise from recessive genes. To get the disease, you have to inherit the bad gene from both parents. That’s more likely when both parents come from the same population.

By mating within a breed, the authors explain, you can suffer a “loss of genetic diversity, thereby increasing the likelihood of recessive disorders within a breed population.” Conversely,

the random mating practices of mixed-breed dogs have been suggested to increase hybrid vigor (heterosis), resulting in healthier dogs. The increased homozygosity expected in purebred dogs offers the potential for these animals to have traits influenced by recessive alleles in greater frequency than their crossbred counterparts.

Other studies largely support this pattern. And the news gets worse for Nugent: The same thing appears to be true in humans.

A year ago, Carl Zimmer, an outstanding science journalist, interviewed Harvard geneticist David Reich for an article in Discover. Reich’s work, Zimmer concluded,

leaves no doubt that interbreeding was a major feature of human evolution. Billions of people carry sizable chunks of DNA from Neanderthals and other archaic human relatives. Some of those genes may play important roles in our health today.

For instance:

In August 2011, Peter Parham of Stanford University and his colleagues found that the Neanderthal and Denisovan versions of some immune system genes are now remarkably widespread. They can be found in Europe, Asia, and even the Pacific islands. Their prevalence suggests that they may have provided some disease-fighting advantage.

That inference remains tentative. But we’re just beginning to unravel our mixed ancestry:

“We’ve been mixing quite often with distant relatives in our history,” Reich says. In fact, he expects much more evidence of interbreeding to surface. There may be other, undiscovered humanlike beings lurking in our genomes.

Top Comment

I'm willing to give Nugent the benefit of the doubt that his lineage is pure. More...

In other words, we’re all subhuman mongrels. And that’s a good thing. Nature favors genetic exchange. We’re the ones who have imposed strict inbreeding. Our fetish for purity, not just in animals but among human races, is, to a considerable extent, an artifact of this perversion.

Be grateful for mongrels, Mr. Nugent. And try to cultivate a healthier level of intellectual cross-fertilization in your own life. Talk to some communists and community organizers. It’s good for you.